1. Field of the Invention
The present invention relates to a cap member for covering a semiconductor chip, and to a semiconductor device employing such a cap member.
2. Description of Related Art
As one type of package for semiconductor laser chips (semiconductor chips) for use in optical pickup devices or the like, there are conventionally known can-package-type semiconductor laser devices (semiconductor devices) in which a semiconductor laser chip is sealed in a metal cap member. Depending on the kind of semiconductor laser chip incorporated in them, some of such can-package type semiconductor laser devices have semiconductor laser chips sealed airtightly in cap members as just mentioned. For example, with nitride-based semiconductor laser chips, when they are driven in the atmosphere, dust may adhere to their laser light exit part, or an organic substance may burn onto their laser light exit part, inconveniently resulting in degraded characteristics. For this reason, nitride-based semiconductor laser chips are generally incorporated in a can package, in a state airtightly sealed in it, when used as light sources in optical pickup devices or the like.
A cap member for sealing a semiconductor laser chip in has formed therein an opening through which to extract laser light. To a rim portion of the opening in the cap member, a light transmission window formed of glass is airtightly fitted by use of low-melting-point glass so as to stop the opening. Such a structure of a cap member is disclosed in, for example, JP-A-2005-101481.
On the other hand, with the recent trend for downsizing and slimming down electronic devices such as notebook-type personal computers, with a view to incorporating into such electronic devices optical disc drives including optical pickup devices, there is demand for slimming down optical disc drives. Concurrent with this trend, there is demand for downsizing can-package-type semiconductor laser devices for use as light sources in optical pickup devices.
However, downsizing can-package-type semiconductor laser devices leads, inconveniently, to degraded heat-dissipation characteristics. With degraded heat-dissipation characteristics, less of the heat generated when a semiconductor laser chip is driven is dissipated, resulting in a higher chip temperature of the semiconductor laser chip. Since this degrades the chip characteristics and reliability of the semiconductor laser chip, an improvement in heat-dissipation characteristic is desired.
Against this background, there is conventionally known a structure of a can-package-type semiconductor laser device that allows an improvement in heat-dissipation characteristics even in the face of downsizing. FIG. 45 is a sectional view illustrating a structure of a conventionally known can-package-type semiconductor laser device. As shown in FIG. 45, the conventionally known can-package-type semiconductor laser device is provided with a stem 3001, a block portion 3002 provided on the stem 3001, a semiconductor laser chip 3004 fitted via a sub-mount 3003 on a side face of the block portion 3002, lead pins 3005 for supplying electric power to the semiconductor laser chip 3004, and a cap member 3100 for airtightly sealing the semiconductor laser chip 3004 in. In this can-package-type semiconductor laser device, with a view to improving heat-dissipation characteristics, the block portion 3002 on which the semiconductor laser chip 3004 is mounted is formed as large as possible. That is, the block portion 3002 functions as a heat sink, and forming this block portion 3002 as large as possible ensures the desired heat dissipation.
The cap member 3100 is formed by press-working of a metal sheet, and includes a cylindrical side wall portion 3101, a top face portion 3102 provided at one end of the side wall portion 3101, and a flange portion 3103 provided at the other end of the side wall portion 3101. In the top face portion 3102 of the cap member 3100, an opening 3102a through which to extract laser light is provided, and the opening 3102a of the cap member 3100 is covered by a light transmission window 3104 to thereby seal the semiconductor laser chip 3004 airtightly in. The light transmission window 3104 is fitted to the cap member 3100 by use of low-melting-point glass 3105.
On the other hand, the flange portion 3103 of the cap member 3100 is formed at the other end of the cylindrical side wall portion 3101 as a result of the metal sheet being bent outward of the side wall portion 3101 with a predetermined radius of curvature. The flange portion 3103 is welded to the upper face of the stem 3001, and as a result the cap member 3100 is fixed to the upper face of the stem 3001 so as to cover the semiconductor laser chip 3004 and the block portion 3002.
Here, in the can-package-type semiconductor laser device shown in FIG. 45, with a view to improving heat-dissipation characteristics, the block portion 3002 is formed as large as possible, and accordingly the diameter D of the cap member 3100 covering the block portion 3002 is made as large as possible, so as to be large enough to cover the block portion 3002.
FIG. 46 is a sectional view illustrating a method for fixing the cap member on the stem in the can-package-type semiconductor laser device shown in FIG. 45. Now, with reference to FIG. 46, the method for fixing the cap member 3100 on the stem 3001 will be described. First, the cap member 3100 is put on the upper face of the stem 3001 so as to cover the block portion 3002 and the semiconductor laser chip 3004. Next, a second electrode 3300 is put in contact with the lower face of the stem 3001, and in addition a first electrode 3200 is moved toward the stem 3001 (in FIG. 46, in the direction of arrow S) so that the first electrode 3200 presses the flange portion 3103 of the cap member 3100 onto the upper face of the stem 3001. Then electric current is passed between the first electrode 3200 and the second electrode 3300. This causes part of the flange portion 3103 to melt under the heat due to electrical resistance, and as a result the flange portion 3103 of the cap member 3100 is welded on the upper face of the stem 3001. In this way, the semiconductor laser chip 3004 is sealed airtightly in the cap member 3100.
However, in the can-package-type semiconductor laser device shown in FIG. 45, as a result of the diameter D of the cap member 3100 being made as large as possible, the distance b from the outer surface of the side wall portion 3101 to one end of the flange portion 3103 is small. Thus, when the first electrode 3200 (see FIG. 46) presses the flange portion 3103 of the cap member 3100 onto the upper face of the stem 3001, inconveniently, the part bent at the predetermined radius of curvature—called the curved-surfaced part (round part) 3106—is pressed. As a result, when the first electrode 3200 presses the flange portion 3103, inconveniently, a force is applied also to a part other than the flange portion 3103, namely also to the side wall portion 3101 and the top face portion 3102.
Since the low-melting-point glass 3105 by use of which the light transmission window 3104 is airtightly fitted is relatively brittle, if a force is applied to the side wall portion 3101 and the top face portion 3102 of the cap member 3100, inconveniently, the force may break the low-melting-point glass 3105, causing the light transmission window 3104 to drop off, or may develop a crack in the low-melting-point glass 3105. This causes the can package to lose airtightness, and thus degrades the chip characteristics etc. of the semiconductor laser chip 3004. Thus the conventional can-package-type semiconductor laser device described above has the problems of low reliability and low fabrication yields.
Incidentally, even if, when the first electrode 3200 presses the flange portion 3103, the low-melting-point glass 3105 does not break, or does not develop a crack, stress remains in the cap member 3100. As a result, when an external force is applied to the cap member 3100 with such stress remaining in it, inconveniently, the low-melting-point glass 3105 easily breaks, or easily develops a crack.
On the other hand, in can-package-type semiconductor laser devices with common package sizes, such as those with stem exterior diameters of 9 mm, 5.6 mm, etc, as distinct from the can-package-type semiconductor laser device shown in FIG. 45, a sufficiently long distance is secured from the outer surface of the side wall portion of the cap member to one end of the flange portion. Thus, when the cap member is welded on the upper face of the stem, as compared with the can-package-type semiconductor laser device shown in FIG. 45, a force is less likely to be applied to the side wall portion and the top face portion of the cap member. Accordingly, when the cap member is welded, the low-melting-point glass by use of which the light transmission window is airtightly fitted is less likely to break or develop a crack.
Inconveniently, however, since the low-melting-point glass by use of which the light transmission window is airtightly fitted is brittle as described above, if an external force is applied to the cap member, even in can-package-type semiconductor laser devices with common package sizes as mentioned above, inconveniently, the low-melting-point glass may break, causing the light transmission window to drop off, or may develop a crack. This results in the problems of low reliability and low fabrication yields of products (can-package-type semiconductor laser devices).